Genome Evolution of the Mitochondrion
Mitochondria originated from within the bacterial phylum Alphapreoteobacteria, and evolved according to the endosymbiont hypothesis. This is the idea that the mitochondrion evolved from a bacterial progenitor through symbiosis. However, it has been found that mitochondrial evolution has taken different pathways in different eukaryotic lineages. In fact, the mitochondrion is often looked at as a genetic and function mosaic, where most of the mitochondrial proteome evolved outside of the Alphaproteobacteria. New research and data questions if mitochondria originated after the eukaryotic cell, as assumed in the endosymbiont hypothesis theory, or if the mitochondrion existed at the same time as the cell it was contained in. Genetic, Genomic and Phylogenetic Data A 16kb mitochondrial genome from several mammalian species was completely sequenced to reveal the genetic function of mitochondria DNA (mtDNA). This showed that mtDNA encodes a small number of protein subunits. Further investigations of mtDNA from non-animal species showed that mtDNA held a higher coding capacity as well ass variation in size, physical form, organizational patterns and modes of expression. There are additional respiratory and ribosomal proteins encoded within mtDNA. Research has also shown that some mitochondrial genomes have decreased in size, losing many of the encoded proteins in the process. Mitochondrial Symbiosis There are two variations, which are two fundamentally different themes, of endosymbiotic models of mitochondria. These are the archezoan scenario and the symbiogenesis scenario. In the archezoan scenario, “The host of the proto-mitochondroal endosymbiotn was hypothetical primitive amitochondrial eularyote, termed archezoan” (Koonin 2010). On the other hand, in the symbiogenesis scenario, “A single endosymbiotic event involving the eptake of an alpha-proeobacterium by an archaeal cell led to the generation of the mitochondria” (Koonin 2010). This was followed “by the evolution of the nucleus and compartmentalization of the eukaryotic cell” (Koonin 2010). The archezoan scenario is closest to the classical endosymbiont hypothesis of mitochondrial evolution. The basic difference between the two scenarios is if the alpha-proteibacterial emdosybiosis that produced the proto-mitichondrion was simultaneous to the formation of the eukaryotic cell, like in the symbiosis scenario, or the primitive amitochondriate cell, like in the archezoan scenario. Two other hypotheses are the endosymbiotic and autogenous hypotheses. Endosymbiotic Hypothesis This hypothesis suggests that mitochondria were originally prokaryotic cells. They were able to implement oxidative mechanisms that eukaryotic organisms could not implement. They lived within eukaryotes, which is why they were known as endosymbionts. Autogenous hypothesis In this hypothesis, it is believed that mitochondria originated by splitting off a portion of DNA from the nucleus of a eukaryotic cell. This should have occurred during the divergence of eukaryotes and prokaryotes. The DNA section was most likely enclosed by membranes, which would not allow proteins to penetrate. Currently, this is the most accredited theory, because mitochondria have many features in common with bacteria. The ribosomes that the mtDNA code for a similar in size and structure, and they resemble bacterial 70S ribosomes. The hypothesis suggests that mitochondria descended from bacteria survived endocytosis and incorporated themselves into the cytoplasm of the cell. They are able to respire in host cells. This depends on glycolysis and fermentation. The symbiotic relationship between mitochondria and eukaryotes most likely developed 1.7 to 2 billion years ago. Archezoan Scenario Archezoa are early branching lineages that are made of protists and are characterized by the absence of a recognizable mitochondrion. It is assumed that they have never had mitochondria in their evolutionary history. This is known as the archezoan hypothesis. Two findings led to the abandonment of this hypothesis. 1) The early branching position of archezoan taxa in the eukaryotic tree is methodological artifact, due to a relatively high rate of sequence divergence of the archezoan sequences used in the analysis. This incorrectly positions these taxa at the base of the eukaryotic clade, which is closest to the prokaryotic sequences used to root the tree. 2) Remnants have been found in every amitochondrial lineage that has been investigated. Therefore, “we currently know of no extant eukaryotic lineages that are convincingly amitochondrial and that there might have been primitively amitochondriate” (Embley and Hirt 1998). This means that lineages without mitochonria have not been discovered, or that they may have become extinct. Therefore, this hypothesis has been rejected. Symbiogenesis Scenario The alternative view is that the host cell for the mitochondrial endosymbiosis was a specifically an archaeon, not a euakryote. This is known as the hydrogen hypothesis (Martin and Muller 1998). This proposes that eukaryotes arose through symbiotic association of a hydrogen-dependent and autotrophic archaebacterium host, where a eubacterium was the symbiont. The eubacterium was able to respire and generate molecular hydrogen as a waste product. This is believed to be the most common ancestor of eukaryotic cells because it is dependent on molecular hydrogen produced by the symbiont (Martin and Muller 1998). The hydrogen hypothesis suggests “the origins of the symbiont and the origins of the eukaryotic lineage are identical” (Martin and Muller 1998). The hydrogen hypothesis predicts that metabolism genes for anaerobic energy were inherited vertically through eukaryotes from a common ancestor. It is also possible that ancestral lineages contributing to a bacterial-archael symbiogenesis might have already contained genomes that were affected by horizontal gene transfer. References 1) http://en.wikipedia.org/wiki/Mitochondrion#Origin 2) http://cshperspectives.net/content/4/9/a011403.full 3) Koonin EV. 2010. The origin and early evolution of eukaryotes in the light of phylogenomics. Genome Biol 11: 209